THE THUNDERSTORM 311
a. Winter convections cannot, in general, rise to nearly so great altitudes nor with such velocity as those of summer.
b. The absolute humidity of summer air may, at times, be greatly in excess of that of winter.
c. The winter snow level usually is much below that of summer. Hence, thunderstorms, since they depend, as explained, upon the
action of strong vertical convection on an abundance of rain drops, necessarily occur most frequently during the warmer seasons, and only occasionally during the colder months. In middle latitudes, where there are no late spring snows to hold back the temperatures, the month of maximum frequency is June. In higher latitudes, where strong surface heating is more or less delayed, the maximum occurs in July or even August.
Yearly Ocean Period.—Over the oceans, on the other hand, temperature gradients favorable to the genesis of thunderstorms, and, therefore, the storms themselves, occur most frequently during winter and least frequently during summer. This is because the temperature of the air at some distance above the surface, being: largely what it was when it left the windward continent while that of the water, and, of ,
relatively but little through the y^._,____ ,
decrease of temperature with Increase of elevation is least, and, therefore, thunderstorms fewest, in summer, and greatest, with such storms most numerous, in winter.
Cyclic Land Period.—Since thunderstorms are accompanied by rain, and since over land they are most numerous during summer, it would appear that they must occur most frequently either in warm or in wet years and least frequently in cold or in dry years. Further, if it should happen, as it actually does, that, for the earth as a whole, warm years are also wet years and dry years cold years, it would appear logically certain that, for the entire world, the maxima numbers of thunderstorms must belong to the years that are wet and warm, and the minima to those that are cold and dry.
A complete statistical examination of these statements is not possible, owing to the fact that meteorological data are available for only portions of the earth's surface and not for the whole of it. Nevertheless, well-nigh conclusive data do exist. The annual rainfall, for instance, to the leeward of a large body of water, obviously must bear the same relation to the annual average windward temperature that the total annual precipitation over the entire world does to the annual average world temperature. In each case the amount of evaporation or amount of water vapor taken into the atmosphere, and, therefore, the amount of subsequent precipitation, clearly must increase and decrease with the temperature. Support of this deduction appears to be furnished by Fig 104, even though it